Studies of gastric secretion of H, pepsin and electrolytes are being done in gastric fistula dogs, some of which have had highly selective vagotomy 3-4 years before. These studies include dose responses to histamine, pentagastrin urecholine and insulin or 2-Deoxy-D-glucose, modified by inhibitors including H-2 antagonists, prostaglandins and atropine. Gastrin release is being studied in these dogs with vagal or cholinergic stimuli, bombesin, CCK, secretin and insulin, and results related to gastric secretion. Pentagastrin dose responses in duodenal and gastric ulcer patients with intact and with cut vagi provide the bases for kinetic calculations of Vmax and Km. Serum gastrin in these patients and in those with Zollinger-Ellison syndrome is being measured fasting and after secretin stimulation. Gastrin species are being determined in the blood by a combination of column chromatographic separation and radioimmunoassay.